(a) Field of the Invention
The present invention relates to an automotive mirror in which an electrochromic element is used, and more particularly to an automotive mirror of which the surface coloration is variable in response to a variation of the voltage applied to the electrochromic element, so-called "nonglaring mirror", of which almost the entire surface can quickly take a predetermined colored state.
(b) Related-art Statement
The automotive mirror which provides the driver of a car with a view behind him is indispensable for safe driving. However, in case one drives in dark, the light from the head lights of a following car will possibly be reflected by the rear view mirror of his car and dazzle him. To reduce such glaring or dazzling light, it has been proposed to color the mirror surface, thereby reducing the reflectance of the mirror when a strong light beam from the head lights of a following car is incident upon the rear view mirror of a car running ahead of the car in the dark, especially, at night. Heretofore, non-glaring mirrors using a prism or liquid crystal have been proposed but they do not fully meet the requirements for the automotive rear view mirrors because of their visibility, difficulty of manufacturing a mirror surface specially curved as required for the automotive rear view mirrors and for some other reasons. Recently, however, an automotive mirror employing an electrochromic element for adjusting the coloration, that is, reflectance, of the mirror surface has been proposed for overcoming the disadvantages of the conventional nonglaring mirrors. The mirror of this type utilizes the fact that the electrochromic element is colored or decolored depending upon the change in orientation of an electric field applied thereto.
FIG. 1 is a schematic, partially fragmentary view of a mirror formed using an ordinary electrochromic element. The electrochromic element may be composed of a transparent conductive layer 2 (formed by a material such as ITO (indium tin oxide) or SnO.sub.2 and composing a first electrode) laminated on one side of a glass substrate 1 so formed as to have an appropriate curvature, a first EC (electrochromic) layer 3 made of a solid electrochromic substance such as WO.sub.3, MoO.sub.3 or the like which colors when deoxidized and which is laminated on the transparent conductive layer 2, a solid electrolyte layer 4 (made of a material such as Ta.sub.2 O.sub.5, ZrO.sub.2, SiO.sub.2 or the like) laminated on the first EC layer 3, a second EC layer 5 made of a solid electrochromic substance such as Cr.sub.2 O.sub.3, NiO, IrO.sub.2 or the like which colors when oxidized and which is laminated on the solid electrolyte layer 4, a second electrode/reflective layer 6 made of a material such as Al, Ag or the like and which is laminated on the second EC layer 5, and a protective layer 7 formed on the second electrode 6 and which is electrically insulative. When a negative DC voltage is applied to the first EC layer 3 through the first electrode 2 of such electrochromic element while a positive DC voltage is applied to the second EC layer 5 through the second electrode 6, an electrochemical deoxidation and oxidation take place in the first and second EC layers 3 and 5, respectively, and the first EC layer 3 is colored light blue in case it is made of WO.sub.3 and the second EC layer 5 is colored light brown in case it is made of Cr.sub.2 O.sub.3. When DC voltages opposite in polarity, respectively, are applied to the EC layers 3 and 5, respectively, reverse electrochemical reactions take place in them, respectively, thereby decoloring them. Based on this principle, the optical reflectance of the mirror surface is adjusted, according to the brightness of the environment, to a lower one when the EC layers are colored, and to the initial one when the EC layers are decolored. The first and second electrodes described in the above are so formed as to have nearly a same shape as or a somewhat smaller shape than the mirror surface, for example, a rectangular shape of about 10 cm by 15 cm. Normally, when a DC voltage of about 0.5 to 2.0 volts is applied between the first and second electrodes, a relatively large current of, for example, 0.8 to 1.0 A, flows for a few seconds because the initial resistance of the first and second EC layers 3 and 5 is relatively low, so that an electrochemical deoxidation and oxidation take place in the first and second EC layers 3 and 5, respectively, while the resistance of each EC layer increases gradually as the charged electricity is accumulated. The lead terminals connecting the layers composing the square electrodes and the positive or negative DC source are so designed as to be led out from a relatively small area at the periphery of each electrode, but each layer forming the electrode has a rather wide area as compared with the lead terminal lead-out portion. Hence, for connection of each electrode to the positive or negative DC source, the current density is small at a position far from each lead terminal and among others, the potential distribution is not uniform on the surface of the transparent electrode 2. This is because the transparent electrode 2 is made of a material such as ITO of which the surface electrical resistance is relatively high, for example, about 20 ohms/cm.sup.2, and the thickness of the electrode may not be more than a predetermined value because of the necessary appropriate transparency, manufacturing costs and adhesion to other adjoining layers. So, the voltage drop is small at a portion near each lead terminal while being large at a portion away from the terminal. Thus, since the degree of coloration of the first and second EC layers 3 and 5 is low in proportion to the distance from each lead terminal, it takes time until the entire mirror surface gets evenly colored after the connection between each electrode and the positive or negative DC source or until the entire mirror surface is evenly decolored after the disconnection between the electrode and positive or negative DC source.